The invention is directed to a peak detector and in particular to a N-diode peak detector for a digital laser transmitter.
In general, an optical transmitter comprises a semiconductor laser, which converts an RF electrical signal to an intensity-modulated optical signal suitable for propagation through fiber optic cables. During this conversion, the laser must be biased at a setting above the stimulated emission threshold referred to as the quiescent DC laser forward bias current. The laser also must be biased at a setting below a maximum power, for keeping the transmission system distortions at a low level.
Typically, a closed loop control circuit is used to maintain a constant quiescent optical output power of the laser. The optical feedback is generally carried out using a photodetector, placed in a suitable location to monitor either the optical power obtained from the rear facet of the laser, or the power emitted from the front facet, or both. The output of the photodetector is an electrical signal, indicative of the power of the monitored optical signal. In typical applications, this electrical signal is then filtered and conditioned via a transimpedance gain stage, developing a voltage proportional in magnitude to the laser optical power output, which is then used to control the bias of the laser.
Peak detectors measure the peak value of an input voltage using a non-linear element, as for example a diode. A peak detector may be connected in the closed loop after the filtering and amplification stages, to measure the amplitude of the output of the laser.
It is an object of the present invention to provide a peak detector for monitoring the output of a digital laser transmitters.
It is another object of the invention to provide a peak detector for input signals with amplitudes comparable with the threshold value of the detector diodes.
Accordingly, the invention provides a device for measuring the peak value Vpk of a small amplitude AC balanced input signal comprising: a non-linear monitoring element for receiving the input signal and providing a detected signal of an average voltage Vavg; a compensation circuit coupled to the monitoring element for converting the average voltage Vavg into a DC output signal proportional with the peak amplitude of the input signal, while tracking the changes in the amplitude of the input signal; and a biasing circuit for forward biasing the monitoring element.
Advantageously, the circuit according to the invention presents a high sensitivity to low input signals, as well as high accuracy over a large temperature range (0-70xc2x0) which are essential to the operation of digital laser transmitters.